This invention relates to a process for separating out clarified water from a water dispersion of particulate solids from the group comprising ash, slag, char, and mixtures thereof as produced by cooling and/or scrubbing with water the hot gas stream from the partial oxidation of ash-containing solid carbonaceous fuels.
Ash containing solid carbonaceous fuels e.g. coal, lignite, oil shale, tar sands have been used as fuel in the partial oxidation process for the production of synthesis gas, reducing gas, and fuel gas. Reference is made for example to coassigned U.S. Pats. Nos. 3,544,291; 3,607,157; 3,976,442; and 3,996,026.
The hot raw gas stream leaving the reaction zone of the gas generator for the partial oxidation of ash-containing solid carbonaceous fuel contains entrained solid particulate matter e.g. slag, ash and char. The temperature of the raw effluent gas stream is in the range of about 1700.degree. to 3000.degree. F. The raw gas stream may be cooled by direct quenching in water. Alternatively, the hot raw gas stream may be partially cooled by indirect heat exchange and then scrubbed with water. Substantially all of the entrained solids are removed from the gas stream during the quench cooling and/or scrubbing operation and a dilute water dispersion is formed. It is of economic importance to remove clarified water from the dispersions of spent quench and/or scrubbing water and recycle the clarified water in the process. Where the concentrated slurries of solid fines are rich in carbon, it is desirable to recycle them to the gas generator as a portion of the feed.
One major problem in recovering and recycling the solid fines from dilute water dispersions is their resistance to settling. This resistance is caused by their natural hydrophobicity and fine particle size. The hydrophobic nature of these solids prevents them from becoming completely wetted. Further, there is a tendency for them to seek out air/water interfaces, either at the surface or in the pockets caused by agitation. The result is a suspension or cloudy water phase from which it is difficult to separate and collect the solid particulate. Recovery becomes expensive. Even for example with vacuum filtration, a substantial amount of the fines e.g. 10-25% remains in suspension. It was unexpectedly found that by the subject process, the particulate matter comprising ash, slag, char, and mixtures thereof in the dilute dispersions of quench and scrubbing water can be made to rapidly separate from the water. Separation times are faster than those in coassigned U.S. Pat. No. 4,525,280 in which more costly surfactants coupled with metal salts are employed. The clarified water may be then recycled to the quench cooling and/or scrubbing zone; and any solid particulates which are rich in carbon may be recycled to the gas generator as a portion of the fuel. The efficiency of the partial process is thereby increased.